Wavelength insensitive integrated optic polarization splitter

ABSTRACT

An integrated optic polarization splitter, includes a pair of waveguide elements with a first waveguide element having horizontal orientation and a second waveguide element having a vertical orientation formed from a plurality of waveguide core layers. The first and second waveguide elements are intersected or nearly intersected at one end of the structure and separated at the other end of the structure and the transition there between is made to be adiabatic. The waveguide elements receive an optical signal having both a TE component and a TM component. The TE component propagates along the horizontally oriented waveguide element and the TM component propagates along the vertically oriented waveguide element.

PRIORITY INFORMATION

This application is the U.S. national stage application of International(PCT) Patent Application Ser. No. PCT/US2003/034607, filed Oct. 30,2003, which claims priority from provisional application Ser. Nos.60/422,413, filed Oct. 30, 2002, and 60/478,767, filed Jun. 16, 2003.The disclosures of these three applications are incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

The invention relates to the field of integrated optic polarizationsplitters, and in particular to an integrated optic polarizationsplitter based on the intersection or near intersection of horizontallyand vertically oriented waveguides.

As the prevalence of fiber optic communication grows, the demand formore intricate processing of optical signals continues to increase.Since integrated optic devices allow for integration of many opticalfunctions on a chip, integrated optic approaches will likely fill thedemand for more intricate optical signal processing. However, in orderto improve the functionality and reduce the cost per function thedensity of components on the chip must increase.

For a given wavelength, the confinement of a mode in a dielectricwaveguide is determined by the contrast between the core and claddingindices, the higher the contrast, the tighter the confinement. Anoutgrowth of tighter confinement is the ability to pack waveguidescloser together and guide light around sharper bends without substantialradiative loss. Since these are the two most critical parametersaffecting device density, it can generally be said that the higher theindex contrast the greater the device density. However, as the indexcontrast increases, the transverse electric (TE) and transverse magnetic(TM) modes propagating in the waveguides begin to exhibit differentcharacteristics. While in a straight section of a square waveguide, theTE and TM modes propagate at the same rate, in a bend the TE and TMmodes propagate at substantially different rates. And, when a pair ofsquare high index contrast (HIC) guides is coupled, the TE and TM modestend to couple at different rates. Since most integrated opticcomponents are sensitive to both propagation velocity and guide-to-guidecoupling, these effects result in polarization dependent performance, aresult that is not compatible with the random polarization stateemanating from the standard single mode fiber used in telecomapplications.

One way to compensate these effects is to use a rectangular waveguidegeometry and alter the aspect ratio of the guide to compensate for thenatural difference in propagation around a bend and/or equalize theguide-to-guide coupling. However, while one or the other of theseeffects may be compensated in this manner for a particular device, asthe index contrast increases it becomes difficult if not impossible tocompensate both simultaneously in a manner that applies to all deviceson the chip.

Another approach for overcoming the polarization sensitivity of HICintegrated optics is to split the random input polarization emanatingfrom the single mode (SM) fiber with a polarizing beam splitter (PBS),couple the outputs to polarization maintaining (PM) fibers, twist one ofthese PM fibers by 90° degrees and couple the two fibers to separatepaths on the integrated optic chip. On each of these paths identicalstructures are used to process the two components independently. At theoutput, these components are recombined by coupling to another pair ofPM fibers, twisting the PM fiber of the path that had not previouslybeen twisted and coupling both fibers to another PBS which has a SMfiber output. While such an approach, commonly referred to as a“polarization diversity” scheme, is feasible, when implemented with bulkoptics it is also cumbersome. Aligning PM fibers is difficult andexpensive. And, in order to preserve signal integrity the path lengthsmust be matched to within at least one-tenth of a bit length (i.e. −2 mmfor 10 Gb/s signals and −0.5 mm for 40 Gb/s signals assuming an index of1.5).

A better approach is to integrate the splitting function of the PBS andthe rotating function of the twisted PM fiber onto the integrated opticchip. Doing so would eliminate the need to align PM fibers and pathlengths could be matched easily through lithography.

Several integrated optic polarization splitters and rotators (orconverters) have been proposed. However, most of the devices proposed todate rely on the coupling of a pair of waveguide modes. Devices based oncoupled modes generally exhibit a wavelength sensitivity resulting fromdifferences in the dispersion of the super-modes propagating in thestructure. Further, such approaches are very sensitive to fabricationerrors. Even slight changes in the waveguide geometries or separationcan have a significant impact on the device performance.

A better way to form a polarization splitter or rotator is to use theprinciple of mode evolution. By making gradual (or adiabatic) changes tothe waveguide geometry, the modes in the guide can be conditioned andthe polarization states separated or rotated. Such an approach onlyrequires that the modes not exchange power which can be assured byproper design of the waveguide and a slow evolution of the structure.Since prevention of mode coupling is a relatively loose requirement,devices based on mode evolution tend to be wavelength insensitive andfabrication tolerant. It has been proposed and demonstrated that apolarization splitter based on mode evolution can be formed, however,this approach has the disadvantage of requiring multiple waveguidematerials.

Generally, it is the object of the present invention to splitpolarization states with an integrated optic device based on theprinciple of mode evolution.

It is a further object of the present invention that when run in reversethe device operate as a polarization combiner.

It is yet another object of the present invention that the device bewavelength insensitive, tolerant to fabrication errors, and require onlya single material system to construct.

These and other objects of the present invention will become apparent tothose skilled in the art from the following detailed description andaccompanying figures.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided anintegrated optic polarization splitter. The polarization splitterincludes an input waveguide element that inputs an optical signal havingTE and TM components. A vertically oriented waveguide element whichincludes a plurality of core layers is coupled to the input waveguideelement and propagates the TM component of the optical signal. Ahorizontally oriented waveguide element is coupled to the inputwaveguide element and propagates the TE component of the optical signal.

According to another aspect of the invention, there is provided a methodof forming an integrated optic polarization splitter. The methodincludes providing an input waveguide element that inputs an opticalsignal having TE and TM components. Moreover, the method includesforming a vertically oriented waveguide element that is coupled to theinput waveguide element and propagates the TM component of the opticalsignal. The vertically oriented waveguide element includes a pluralityof core layers. Furthermore, the method includes forming a horizontallyoriented waveguide element that is coupled to the input waveguideelement that propagates the TE component of the optical signal.

According to another aspect of the invention, there is provided anintegrated optic polarization splitter. The integrated opticpolarization splitter includes a pair of waveguide elements with a firstwaveguide element having a horizontal orientation and a second waveguideelement having a vertical orientation formed from a plurality ofwaveguide core layers. The first and second waveguide elements areintersected or nearly intersected at one end of the structure andseparated at the other end of the structure so that the transition ismade to be adiabatic. The waveguide elements receive an optical signalhaving both a TE component and a TM component. The TE componentpropagates along the horizontally oriented waveguide element and the TMcomponent propagates along the vertically oriented waveguide element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a polarization splitter in accordancewith the invention;

FIGS. 2A–2B are schematic diagrams of mode scattering calculations ofthe TE and TM fields propagating in the polarization splitter depictedin FIG. 1; and

FIGS. 3A–3B are graphs demonstrating the performance of the polarizationsplitter depicted in FIG. 1; and

FIGS. 4A–4C are schematic diagrams of polarization splitters utilizingthree core layers with a gap between the middle core layers of thevertically and horizontally oriented waveguide elements; and

FIG. 5 is a schematic diagram of a polarization splitter utilizing onlytwo core layers with a gap between the middle core layers of thevertically and horizontally oriented waveguide elements; and

FIGS. 6A–6B are graphs demonstrating the performance of the polarizationsplitter depicted in FIG. 5; and

FIGS. 7A–7B are schematic diagrams of polarization splitters utilizingtwo core layers with alternate transition regions into the structure.

DETAILED DESCRIPTION OF THE INVENTION

The polarization splitter of the invention is constructed from theintersection or near intersection of a pair of waveguides. The zone inwhich the waveguides are in closest proximity is the splitter input andthe zone in which they are at their greatest separation is the splitteroutput. For the device to efficiently separate the polarization states,the fundamental TE (or quasi TE) mode of the combined structure at thedevice input must evolve into the fundamental mode of one of the guides,denoted the TE guide, while the fundamental TM (or quasi TM) modeevolves into the fundamental mode of the other guide, denoted the TMguide. For this to occur, the TM mode of the TM guide must be morestrongly guided (have a higher effective index) than the TM mode of theTE guide. Similarly, the TE mode of the TE guide must be more stronglyguided than the TE mode of the TM guide.

The evolution of the waveguide acts as a perturbation to the modestructure inducing coupling amongst the modes in the system, the fasterthe evolution, the stronger the coupling. In order to ensure that thestructure acts as a polarization splitter, coupling amongst thefundamental modes and between the fundamental modes and other modes inthe system must be inhibited. The full spectrum of modes in thestructure consists of the fundamental guided TE and TM modes, unguided(or radiation) modes, and secondary guided TE and TM modes that appearas the horizontally and vertically oriented sections separate. Theunguided modes propagate at substantially faster rates than thefundamental modes. As a result, when the transition is sufficientlyweak, the modes have a chance to de-phase before substantial powerexchange occurs. Therefore, power exchange between fundamental modes andradiation modes can be substantially reduced by making the transitionslow (or adiabatic). Coupling between fundamental modes and secondaryguided modes can be similarly inhibited by ensuring that the secondarymodes also propagate at substantially higher rates. This can be done byforming the structure from a pair of guides with principal axes that areorthogonally oriented. In the electromagnetic sense, the principal axisis defined by the electric field polarization of the fundamental mode ofthe waveguide. For a rectangular buried waveguide, the principal axis isalong the larger of the two dimensions that define the rectangle. In thepresent description, when specifying an orientation of a waveguide,reference is made to the principal axis of the waveguide. For example,the expression “vertically oriented waveguide section” is meant toindicate a waveguide section having a principal axis that is vertical,i.e., orthogonal to a main planar surface (such as the substratesurface) of the waveguide device, while the expression “horizontallyoriented waveguide section” is meant to indicate a waveguide sectionhaving a principal axis that is horizontal, i.e., parallel to a mainplanar surface (such as the substrate surface) of the waveguide device.

Finally, coupling amongst the fundamental modes can be inhibited byfurther ensuring that the fundamental modes propagate at different ratesand/or by positioning the guides in such a manner to prevent couplingthrough mode symmetry.

As a final generality it is important to note that a device which actsas an effective polarization splitter, will through the principle ofreciprocity, act as an effective polarization combiner when run inreverse.

Practical implementations of the device typically require that it beformed from micro-fabrication techniques, which generally require thestructures be formed from a layering process with features definedthrough lithography. It is therefore desirable to build the structurewith as few layers as possible. Herein, a layer is defined as ahorizontal slice through the waveguide cross-section which contains novariations of refractive index in the vertical direction.

The optical waveguides forming the inventive polarization splitter aretypically formed by dielectric materials of various refractive indices.Generally, the higher index materials are considered core materialswhile the lower index materials are considered cladding materials. To bespecific, a cladding material is herein defined as the material oflowest refractive index within a layer. All other materials within thelayer are therefore core materials. A core layer is defined as a layercontaining a core material.

The basic requirements for the structure to operate as a polarizationsplitter are quite loose, with the primary requirement being that thestructure be formed from the intersection or near intersection of a pairof orthogonally oriented waveguides which separate thereby splitting theorthogonally oriented modes into the respective orthogonally orientedwaveguide sections. A few of the possible geometries are describedbelow.

FIG. 1 is a schematic diagram of a polarization splitter 2 in accordancewith the invention. The splitter 2 begins as a pair of orthogonallyoriented rectangular waveguides 14 which are centrally intersected andthen gradually separated into a pair of rectangular waveguides 10, 12,one with a horizontal orientation 10, and the other with a verticalorientation 12, with a final separation of s as shown in FIG. 1. Acladding, with a lower refractive index than the core layers typicallysurrounds the core layers to provide light confinement. The polarizationsplitter in FIG. 1 uses centrally intersected waveguides to inhibitcoupling amongst the fundamental guided modes. As a result, thestructure will typically require a minimum of three core layers 4, 6,and 8 with heights h₁, h₂, and h₃ where h₁ and h₃ are preferablydesigned to be equal. The horizontally oriented waveguide 10 has a widthw₂ and height h₃, and the vertically oriented waveguide 12 has a widthw₁ and height that is the sum of h₁, h₂, and h₃. At the input of thepolarization splitter only two guided modes exist, a fundamental TE modeand a fundamental TM mode. At large separations of the horizontallyoriented and vertically oriented sections, the fundamental TE mode isalmost entirely confined to the horizontally oriented section and thefundamental TM mode is confined to the vertically oriented section.Thus, the natural evolution of the fundamental modes results in asplitting of the TE and TM components.

It is important to note that many variations of the described embodimentare possible. The waveguides need not be rectangular in geometry and thecore layers need not have the same refractive indices or geometry.

FIGS. 2A–2B are schematic diagrams of mode-scattering simulations of TEand TM fields propagating in the polarization splitter of FIG. 1. Themode scattering technique takes overlaps between the local modes at eachcross-section along the length of the structure and propagates the fieldbetween cross-sections. Since a reduced set of modes are typically usedto minimize the calculation time, mode scattering simulations are aparticularly useful modeling tool only when a few modes per waveguidecross-section are required to represent the system. Since the radiationmodes do not substantially influence the operation of approaches basedon mode evolution, the mode-scattering technique is well suited forthese problems. In the embodiment used for these simulations, the corerefractive index is 2.2 and the cladding index is 1.445. The dimensionsof the horizontally and vertically oriented waveguide cores are0.25×0.75 μm and 0.75×0.25 μm, respectively, indicating a layerthickness of 0.25 μm. The length of the splitter is 30 μm and thedistance separating the horizontally oriented rectangular waveguide 22and vertically oriented rectangular waveguide 20 is 1 μm at the deviceoutput. However, other dimensions can be used in other embodiments.

FIG. 2A shows the TE field propagating in the splitter 2. In particular,the TE field propagates in the horizontally oriented rectangularwaveguide 22 and not the vertically oriented rectangular waveguide 20.

FIG. 2B shows the TM field propagating in the splitter. The TM fieldpropagates in the vertically oriented rectangular waveguide 20 and notthe horizontally oriented rectangular waveguide 22. Thus, FIGS. 2A–2Bdemonstrate the ability of the splitter to separate TE and TM componentsof a randomly polarized input signal.

FIGS. 3A–3B are graphs of mode-scattering and full three dimensionalfinite difference time domain (FDTD) simulations, respectively,demonstrating the performance of the polarization splitter depicted inFIG. 1. Here again, the core refractive index is 2.2 and the claddingindex is 1.445. The dimensions of the waveguide cores are 0.25×0.75 μmand 0.75×0.25 μm, respectively, indicating a layer thickness of 0.25 μm.The waveguide elements are separated at the output by a distance s=1 μm.FIG. 3A shows the relationship between the length of the inventivepolarization splitter and the normalized output modal power for the TEand TM modes. In particular, FIG. 3A shows that for lengths that arelarger than 25 μm, the normalized output modal power for both the TE₁₁(fundamental TE) mode and TM₁₁ (fundamental TM) modes are nearly 1 withvery little cross-talk (TE₁₁ to TE₂₁ and TM₁₁ to TM₂₁ coupling) over theentire 1.45 μm to 1.65 μm band. The performance of the inventivesplitter improves as the transition becomes more adiabatic.

FIG. 3B demonstrates the wavelength insensitive nature of the devicefrom 1.45 μm to 1.65 μm) which includes the telecom wavelengths using afull three-dimensional FDTD simulation. The FDTD method is a numericalimplementation of Maxwell's equations with the only errors being thosecaused by the grid discretization. In contrast to the mode-scatteringtechnique all modes of the system are taken into account. For thepresent simulation the device length is 25 μm. In this range, thenormalized output modal power for both the TE₁₁ mode and TM₁₁ modes arenearly 1 with very little cross-talk (TE₁₁ to TE₂₁ and TM₁₁ to TM₂₁coupling) over the entire 1.45 μm to 1.65 μm band. This indicates thatthe inventive splitter device does not possess any significantwavelength sensitivity in the telecom wavelength regime.

FIGS. 4A–4C are diagrams of polarization splitters 100, 102, and 104 inwhich the vertically 106, 108, and 110 and horizontally orientedwaveguides 112, 114, and 116 do not have a point of intersection.Although in theory the performance of the device depicted in FIG. 1 isnearly ideal, when fabricated some rounding may occur in the regionwhere the two waveguides intersect. This rounding will only occur in themiddle layers 118, 120, and 122 and is a result of the limitedresolution of optical lithography.

However, the impact on performance may be substantial as this would leadto a rather abrupt junction in the waveguides 10 and 12 of FIG. 1.Hence, it would be desirable to remove the intersection point. This canbe accomplished by keeping the middle layers 118, 120, 122 of thevertically oriented waveguides 106, 108, and 110 separated from thehorizontally oriented waveguides 112, 114, and 116 by a small gap s₁. Solong as the gap s₁ is greater than the resolution limit of thelithographic system, the fabrication error will be removed. Note thatthe dimensions of the vertically oriented waveguide 106, 108, and 110and horizontally oriented waveguide 112, 114, and 116 are similar tothose described for vertically oriented waveguide 12 and horizontallyoriented waveguide 10 in FIG. 1. Note that the vertically oriented 106,108, and 110 and horizontally oriented waveguides 112, 114, and 116 areseparated by a distance s₂.

FIGS. 4A–4C demonstrate a few of the many ways in which to transitioninto the inventive polarization splitter with a gap between the guidesin the middle layers 118, 120, and 122. In particular, FIG. 4A tapersthe vertically oriented waveguide 106 to transition the input modes intothe polarization splitter adiabatically. In FIG. 4B the various piecesof the core in the layers 130, 120, 134 forming the vertically orientedwaveguide 108 are separately and adiabatically brought into proximitywith the horizontally oriented waveguide 114 so as to ensure that boththe fundamental TE and TM modes originate in the horizontally orientedwaveguide 114. Finally, in FIG. 4C, a reduced width vertically orientedwaveguide 110 is brought into proximity with the horizontally orientedwaveguide 116 and subsequently tapered into the full width structureagain assuring that the fundamental modes originate in the inputhorizontally oriented waveguide 116.

All of these approaches work on the same principle. The modes of theinput waveguide must be adiabatically transitioned in the inventivepolarization splitter wherein the orthogonally oriented waveguides arein close proximity. The approach taken will typically depend on thefabrication technology utilized. These geometries represent just a fewof the many possible ways of coupling to the inventive polarizationsplitter. The waveguide sections need not be rectangular in geometry andthe core layers need not have the same refractive indices or geometry.

FIG. 5 shows a polarization splitter 54 which requires only two corelayers 60, 62 to fabricate. In this embodiment, the vertically oriented58 and horizontally oriented 56 waveguides are no longer centrallyintersected. As a result, the fundamental TE and TM modes couple to oneanother. However, this coupling may again be mitigated by ensuring thefundamental modes propagate at different rates and have a chance tode-phase before substantial power exchange occurs. This is accomplishedby making the horizontally oriented 56 and vertically oriented 58waveguides different sizes. The performance of the device is unaffectedby the ordering of the layers (i.e. which layer sits on top). Note thatthe two core layers 60, 62 have heights h₁, h₂. The structure alsoleaves a gap s₁ between the orthogonally oriented waveguides at theinput to facilitate fabrication. At the output the vertically orientedwaveguide 58 and horizontally oriented waveguide 56 are separated by adistance s₂. In addition, the horizontal waveguide 56 has a width w₂ andheight h₂, and the vertical waveguide 12 has a width w and height thatis the sum of h₁ and h₂.

It is important to note that many variations of the described embodimentare possible. The waveguide sections need not be rectangular in geometryand the core layers need not have the same refractive indices orgeometry.

FIGS. 6A–6B are graphs of mode-scattering and FDTD simulations,respectively, of the performance of the device depicted in FIG. 5. Inthis particular embodiment, the core refractive index is 2.2 and thecladding index is 1.445. The layer thicknesses are each 0.4 μm and theguide widths are 0.35 μm and 0.8 μm for the vertically and horizontallyoriented waveguides, respectively. The input and output separations ofthe guides are chosen to be s₁=0.25 μm and s₂=1.0 μm, respectively.

In particular FIG. 6A shows the performance of the device depicted inFIG. 5 as a function of the device length at a wavelength of 1.55 μm.The graph shows that for lengths over 150 μm, the performance of thistwo layered polarization splitter is nearly ideal. FIG. 6B demonstratesthe performance of the device depicted in FIG. 5 as a function ofwavelength for a device length of 143 μm. FIG. 6B shows that the deviceis largely wavelength insensitive with very little cross-talk (TE₁₁ toTE₂₁ and TM₁₁ to TM₂₁ coupling) over the entire 1.45 μm to 1.65 μmregime.

FIGS. 7A–7B demonstrate a couple of the many ways in which to transitioninto the inventive two layer polarization splitter with a gap s₁ betweenthe guides in the middle layers 82, 84. The approaches are analogous tothose taken in a three layer device. In FIG. 7A, a reduced widthvertically oriented waveguide 68 is brought into proximity with thehorizontally oriented waveguide 66 and subsequently tapered into thefull width structure. In FIG. 7B the various layers 72, 74 forming thevertically oriented waveguide 76 are separately and adiabaticallybrought into proximity with the horizontally oriented waveguide 80.Again, each of these approaches has advantages and disadvantages withrespect to fabrication, but all work on the same principle. The modes ofthe input waveguide must be adiabatically transitioned into theinventive polarization splitter by gradually bringing the orthogonallyoriented waveguides into close proximity. The approach taken willtypically depend on the fabrication technology utilized. Thesegeometries represent just a few of the many possible ways of coupling tothe inventive polarization splitter. The waveguide sections need not berectangular in geometry and the core layers need not have the samerefractive indices or geometry.

Importantly, the principle of reciprocity ensures that all of theaforementioned embodiments will act as polarization combiners when runin reverse.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. An integrated optic polarization splitter comprising: an inputwaveguide element that is configured to receive in input an opticalsignal having TE and TM components; a geometrically vertically orientedwaveguide element coupled to said input waveguide element that isconfigured to propagate said TM component of said optical signal, saidvertically oriented waveguide element including a plurality of corelayers; and a horizontally oriented waveguide element coupled to saidinput waveguide element that is configured to propagate said TEcomponent of said optical signal.
 2. The integrated optic polarizationsplitter of claim 1, wherein said vertically oriented waveguide elementand said horizontally oriented waveguide element intersect or nearlyintersect in correspondence to the input waveguide element andadiabatically separate along the length of the polarization splitter. 3.The integrated optic polarization splitter of claim 1, wherein saidplurality of core layers consists of two core layers.
 4. The integratedoptic polarization splitter of claim 3, wherein said vertically orientedwaveguide element and horizontally oriented waveguide element havedifferent sizes.
 5. The integrated optic polarization splitter of claim1, wherein said plurality of core layers comprises no more than threecore layers.
 6. The integrated optic polarization splitter of claim 1,wherein said vertically oriented waveguide element is tapered.
 7. Theintegrated optic polarization splitter of claim 1, wherein saidvertically oriented waveguide element and horizontally orientedwaveguide element are rectangular waveguides.
 8. The integrated opticpolarization splitter of claim 1 wherein the cross section of thepolarization splitter taken at a point along the length of thepolarization splitter is not equal to the cross section of thepolarization splitter taken at any other point along the length of thepolarization splitter.
 9. A method of forming an integrated opticpolarization splitter, said method comprising: providing an inputwaveguide element that is configured to receive in input an opticalsignal having TE and TM components; and forming a geometricallyvertically oriented waveguide element coupled to said input waveguideelement that is configured to propagate said TM component of saidoptical signal, said vertically oriented waveguide element includes aplurality of core layers; and forming a horizontally oriented waveguideelement coupled to said input waveguide element that is configured topropagate said TE component of said optical signal.
 10. The method ofclaim 9, wherein said vertically oriented waveguide element and saidhorizontally oriented waveguide element intersect or nearly intersect incorrespondence to the input waveguide element and adiabatically separatealong the length of the polarization splitter.
 11. The method of claim9, wherein said plurality of core layers consists of two layers.
 12. Themethod of claim 9, wherein said plurality of core layers comprises nomore than three layers.
 13. An optical waveguide splitter comprising: apair of waveguide elements with a first waveguide element having ahorizontal orientation and a second waveguide element having ageometrically vertical orientation formed from a plurality of waveguidecore layers, wherein said first and second waveguide elements areintersected or nearly intersected at one end of the structure andseparated at the other end of the structure with the transition therebetween made to be adiabatic; said waveguide elements being configuredto receive an optical signal having both a TE component and a TMcomponent and to propagate said TE component along the horizontallyoriented waveguide element and said TM component along the verticallyoriented waveguide.